Fix track matching

I had previously tested the `munkres` -> `lapjv` replacement
extensively, so I was today surprised to find that nothing gets matched
correctly when I tried importing some new tracks.

On the other hand I now remember making a small adjustment in the logic
to make autotagging tests pass which is when I introduced a bug: I did
not realize that `lapjv` returns index '-1' for each unmatched item.

This issue did not get caught by tests because this 'unmatched' item
index '-1' anecdotally ended up pointing to the last (expected) item in
the test making it pass.

This commit adjusts the aforementioned test to catch this issue and
fixes the logic to correctly identify unmatched tracks.

beets/autotag/match.py

@@ -127,15 +127,21 @@ def assign_items(
     objects. These "extra" objects occur when there is an unequal number
     of objects of the two types.
     """
+    log.debug("Computing track assignment...")
     # Construct the cost matrix.
     costs = [[float(track_distance(i, t)) for t in tracks] for i in items]
-    # Find a minimum-cost bipartite matching.
-    log.debug("Computing track assignment...")
-    cost, _, assigned_idxs = lap.lapjv(np.array(costs), extend_cost=True)
+    # Assign items to tracks
+    _, _, assigned_item_idxs = lap.lapjv(np.array(costs), extend_cost=True)
     log.debug("...done.")
 
-    # Produce the output matching.
-    mapping = {items[i]: tracks[t] for (t, i) in enumerate(assigned_idxs)}
+    # Each item in `assigned_item_idxs` list corresponds to a track in the
+    # `tracks` list. Each value is either an index into the assigned item in
+    # `items` list, or -1 if that track has no match.
+    mapping = {
+        items[iidx]: t
+        for iidx, t in zip(assigned_item_idxs, tracks)
+        if iidx != -1
+    }
     extra_items = list(set(items) - mapping.keys())
     extra_items.sort(key=lambda i: (i.disc, i.track, i.title))
     extra_tracks = list(set(tracks) - set(mapping.values()))

test/test_autotag.py

@@ -551,7 +551,7 @@ def test_order_works_with_invalid_track_numbers(self):
     def test_order_works_with_missing_tracks(self):
         items = []
         items.append(self.item("one", 1))
-        items.append(self.item("three", 3))
+        items.append(self.item("two", 2))
         trackinfo = []
         trackinfo.append(TrackInfo(title="one"))
         trackinfo.append(TrackInfo(title="two"))
@@ -560,8 +560,8 @@ def test_order_works_with_missing_tracks(self):
             items, trackinfo
         )
         assert extra_items == []
-        assert extra_tracks == [trackinfo[1]]
-        assert mapping == {items[0]: trackinfo[0], items[1]: trackinfo[2]}
+        assert extra_tracks == [trackinfo[2]]
+        assert mapping == {items[0]: trackinfo[0], items[1]: trackinfo[1]}
 
     def test_order_works_with_extra_tracks(self):
         items = []